| Myrosinase (thioglucoside glucohydrolase; EC 3.2.3.1) is an enzyme present in all glucosinolate-containing plants. Brassicas, when damaged, release glucosinolates which are then hydrolyzed by myrosinase to D-glucose and allelochemicals. The allelochemicals have potential to inhibit weed seed germination and pathogens in soil and Brassicas have thus been suggested as a cover crop. Although myrosinase is the key factor in controlling allelopathic activity derived from glucosinolates, no assay has been developed to measure its activity in soil. Parameters that affect myrosinase activity, persistence, and distribution in soil also have not been investigated. We developed a method to assay myrosinase activity in soil, which involves colorimetric determination of the glucose produced when soil is incubated with a buffered solution (pH 7) of sinigrin (20 mM) for 4h. We found that loss of initial myrosinase activity in three different air-dried soils, stored for seven weeks, ranged from 16.9 to 41.6%. Myrosinase was less stable in field-moist soil than in air-dried soil. Myrosinase activity in soil was associated with Brassica roots and significantly decreased with distance from Brassica stems and with soil depth. Even though glucosinolate products released from Brassica have broad spectrum antimicrobial activity, results showed that Brassica-released glucosinolates did not inhibit the ability of arbuscular mycorrhizal to colonize maize or soybean roots and did not inhibit the formation of active nodules on soybean roots by Bradayrhizobium japonicum. Although shoot dry weights and phosphorus and nitrogen concentrations were significantly reduced in 35-day old maize grown following Brassica, no effects were observed on these measurements in 70-day old maize. Counts of fluorescent pseudomonads and aerobacteria in rhizospheres of 35-days old maize and soybean were significantly decreased when plants were grown following Brassica compared to control. Fungi counts, however, were not affected in these rhizospheres. We conclude that Brassica effects on the subsequent crop are short-lasting. Brassica is recommended as a cover crop provided the subsequent crop is planted 1--2 weeks after killing the Brassica cover crop. |
